Combining in an additive or synergetic manner the most potent strengthening mechanisms available in an alloy is the art of the metallurgist. The various models proposed in the literature in order to interpret the Hall-Petch relation are critically reviewed by comparison with experimental data. The pile-up models and the work hardening theories must include the inner structure of the grain in the case of alloys hardened by a second phase. Similarly, the properties and structure of the grain boundaries are influenced by impurities or the presence of particles. Ultra-fine grain sizes can provide ductility to high strength materials when surface preparation eliminates microcracks. In steady-state creep equations, introducing the influence of grain size in complex alloys by incorporating the Hall-Petch stress as one component of the internal stress helps in rationalizing the existence of an optimal grain size where creep resistance is maximized. Slower crack growth rates can be obtained by controlling the grain boundary structure as well as grain size. Fatigue tests at room temperature clearly point out the interest of small grain sizes for reducing crack initiation, usually associated, however, with lower propagation threshold and somewhat faster growth rates.

Influence of grain size on the mechanical behaviour of some high strength materials

Nanocomposite materials consisting of a nanocrystalline Ni matrix (grain size 10–15 nm) reinforced with sub-micron size SiC particulates (average particle size: 0.4 μm) up to 10.5 vol.% have been produced by pulse electrodeposition. Substantial improvements in mechanical properties including hardness, yield and tensile stress were obtained for the nanocomposite material, as compared with conventional Ni–SiC composites with a matrix grain size in the micrometer range. Tensile strengths up to four times that for conventional polycrystalline Ni and two times that for conventional polycrystalline Ni–SiC of comparable SiC content was measured. The tensile and yield strengths of the nanocomposite material with SiC content less than 2 vol.% were higher than those for pure nanocrystalline Ni of comparable grain size. For these nanocomposites an unexpected increase in tensile ductility was also observed when compared to pure nanocrystalline nickel. At higher SiC content (>2 vol.%) the strength and ductility were found to decrease to the detriment of the nanocomposite. Particle clustering was considered the main cause of this decrease.

Mechanical properties of nickel silicon carbide nanocomposites

On the fracture toughness of cemented carbides

Void growth and ductile failure in the nonuniform multiaxial stress fields of notched bars are studied numerically and experimentally. U-notched bars with different notch acuities are made from partially consolidated and sintered iron powder compacts with various residual porosities. The materials are modelled using an elastic-viscoplastic constitutive relation that accounts for strength degradation resulting from the growth of microvoids. The matrix stress-strain relation and the initial void volume fractions used in the calculations are determined experimentally. The remaining parameters in the constitutive equations are evaluated from micromechanical models. Comparisons of the calculations with experimental results indicate that the constitutive model can provide good estimates of the evolution of the void volume fraction and of the strength reduction induced by void growth under a variety of nonuniform stress histories.

Void growth and failure in notched bars

The ductile fracture of metals: A microstructural viewpoint

Evidence for lattice-diffusion controlled superplasticity in a SnPb-eutectic

Phase boundaries as obstacle to dislocations motion

Crack tip plasticity is simulated as motion of discrete dislocations. The results of the dislocation model are compared with the predictions of a continuous description of crack tip plasticity in the case of a stationary mode I fatigue crack. It is shown that both the dislocation model and the continuum mechanics lead to the same result at high loading levels but they differ significantly for small stress intensity ranges.

A comparison of a discrete dislocation model and a continuous description of cyclic crack tip plasticity

The precipitation behavior of metals is commonly demonstrated in continuous or in isothermal time-temperature transformation diagrams Best known are such diagrams for steel with carbon concentrations above 02 wt% Isothermal TTT''-diagrams are used to show the aging properties of aluminium alloys Conventional methods to establish TTT-diagrams comprise the analysis of the microstructure, strength measurements, dilatometric observations and the measurement of the electrical resistivity All these methods measure directly the results of precipitation Except for the last of these methods, all others may fail to work when the amount of the precipitated phase is too small for detection A very sensitive way to assess the state of precipitation in an interstitional alloy with a bcc structure is measuring the internal friction at the Snoek peak which is caused by the anelastic relaxation of interstitially dissolved atoms Complementary structure analysis should be used to distinguish the different precipitates of one interstitial A further way of distinction, when precipitates form directly from still dissolved interstitial atoms, is to observe the precipitation kinetics However, any later structural changes of the first precipitate are not recognized unless they are accompanied by intermittent reversion phenomena An attempt to produce a TTT-diagram by internal friction may bemore » worthwhile when all other methods are not accurate enough because the carbon concentration is too low as, for instance, in low-carbon steels The steel used to study the precipitation behavior contained, in wt%, 0043 C, 002 Si, 022 Mn, 0008 P, 0012 S, 0061 Al, 002 Cr, 0027 Cu and 002 Ni« less

Hein Peter Stüwe

Papers

Evidence for lattice-diffusion controlled superplasticity in a SnPb-eutectic

Phase boundaries as obstacle to dislocations motion

A comparison of a discrete dislocation model and a continuous description of cyclic crack tip plasticity

The isothermal precipitation behaviour in a low-carbon steel at low temperatures investigated by internal friction

The decrease of dislocation density during cyclic deformation of prestrained copper single crystals